1. Technical Field
The present invention relates generally to shielding from electromagnetic wave transmission and reduction in electromagnetic wave reflection and, more specifically, to dissipating electromagnetic waves using impedance sheets. More particularly still, the present invention relates to absorbing electromagnetic waves, regardless of their angle of incidence, using directionally-shaped impedance sheet absorbers.
2. Description of the Related Art
The ability to attenuate electromagnetic waves incident on a surface material at any angle is useful in many applications. One application is in an anechoic testing chamber used to minimize any reflective electromagnetic waves that would distort the results of the device under test in the chamber. Attenuating material is placed on the floor, the walls, and the ceiling to attenuate any reflective waves interfering with the test results.
It is also useful to use attenuating material to place on an object that is desired to be undetectable to searching electromagnetic waves, such as by radar. This is also desirable to minimize interference with a directional receiver used by the object.
Previous solutions have included using planar impedance sheets that use a thin film of conductive material or using an electrically thick layer of electrically or magnetically lossy bulk materials. Additionally, these planar devices have been stacked one upon another in an attempt to increase the absorption bandwidth. Also, carbon loaded core and pyramidal carbon loaded foam have been used as absorbing devices.
Unfortunately, these prior solutions have had limited success. For example, the use of thin planar impedance sheets provides only poor absorption for certain directions and polarizations of the incident electromagnetic radiation. Furthermore, the absorption of impinging electromagnetic waves by electrically thick planar layers of electrically or magnetically lossy bulk materials is inherently limited by the departure of the material's permitivity and permeability from that of air. Also, the granular nature of many absorbers (such as foams) have poor performance in the higher range of electromagnetic frequencies at which the granular nature of the underlying material can be seen by the shorter wavelengths. Additionally, those sheets made from magnetically lossy material are not lightweight, which is desirable in many electromagnetic absorption situations. In addition, pyramidal foam absorbers are bulky, structurally weak, and not easily incorporated into composite structure. Furthermore, all of these prior solutions have poor absorbing properties near grazing incidence.
Accordingly, what is needed is an apparatus for attenuating incident electromagnetic radiation for all polarizations and directions of incidence without using thick material layers or magnetic materials. Further, what is needed is an apparatus for attenuating electromagnetic radiation that is lightweight in construction and covers a broadbanded absorption range, including good absorption near grazing incidence.